1. Field of the Invention
The present invention relates to a process of producing a vinyl chloride polymer, and more particularly to a process of producing a vinyl chloride polymer which is suitable for producing a vinyl chloride polymer in high quality and high productivity while preventing deposition of polymer scale.
2. Description of the Prior Art
A vinyl chloride polymer is generally produced by suspension polymerization of vinyl chloride monomer or a mixture of vinyl type monomers mainly made up of vinyl chloride monomer in an aqueous medium in the presence of an oil-soluble polymerization initiator. Generally, where a polymer having an average degree of polymerization of 1,700 or more is produced, this suspension polymerization is carried out under such polymerization conditions that the polymerization temperature is 30.degree.to 45.degree. C. and the polymerization time is about 8 hours, whereas where a polymer having an average degree of polymerization of about 700 to 1,000, the suspension polymerization is carried out under such polymerization conditions that the polymerization temperature is 55.degree. to 63.degree. C. and the polymerization time is about 4 hours. At that time, the oil-soluble polymerization initiator is normally used in an amount of about 0.04 to 0.20% by weight based on the monomer, although the amount varies depending on the type of the oil-soluble polymerization initiator to be used. Depending on the type of the polymerization initiator used at that time, the polymerization initiator affects greatly the pattern of heat generation, the polymerization time, the productivity, and the deposition of polymer scale during polymerization as well as the initial coloration of the polymer and the quality and characteristics of the polymer, such as the heat stability, the odor, and the anti-leaching.
In recent years, it is attempted to shorten the polymerization time in order to improve the productivity of vinyl chloride polymers. As one technique of shortening the polymerization time, it is thought to increase the amount of the polymerization initiator to be added. However, where the amount of the polymerization initiator is increased, the following problems arise:
(a) Polymer scale is deposited within a polymerization vessel. PA1 (b) Since the amount of the heat generation of the reaction per unit time at the time when the amount of the heat generation of the reaction per unit time reaches the peak during the polymerization is increased considerably, the cooling capacity of the existing polymerization apparatus becomes insufficient at the time of this peak. Accordingly, the increase in the amount of the polymerization initiator is limited and therefore the shortening of the polymerization time is also restricted. PA1 (c) Either the quality of formed products of the resulting vinyl chloride polymer is lowered: for example, the fish eyes are increased, the initial coloration is degraded, and the heat stability is lowered, or an increase in the residue and decomposed products of the polymerization initiator causes such problems as generation of a nasty odor, an increase in leaches, and an increase in deposition of polymer scale in the polymerization vessel. PA1 (A) an organic peroxide such that a 10-hour half-life period temperature of a solution containing 0.1 mol of the organic peroxide per liter of benzene is 30.degree. to 50.degree. C. and the organic peroxide does not, in its structure, contain a benzene ring or an alkoxy group, and PA1 (B) 3-hydroxy-1,1-dimethylbutyl peroxyneodecanoate, in combination.
To cope with these problems, a technique is suggested wherein two types of oil-soluble polymerization initiators are used in combination. Specifically, as this technique, it is suggested to use BPD (t-butyl peroxyneodecanoate) and CPD (.alpha.-cumyl peroxyneodecanoate) in combination. According to this technique, although it is possible to make up the insufficient cooling capacity and to shorten the polymerization time, there are such defects as generation of polymer scale deposition within the polymerization vessel at the end of polymerization an increase in fish eyes of the obtained vinyl chloride polymer, generation of a nasty odor due to an increase in the residue and decomposed products of the polymerization initiators, an increase in leaches, and a deterioration in the quality of the polymer.
As a technique capable of obviating the defects of the above technique, it is proposed to use BPD (t-butyl peroxyneodecanoate) and HDPH (3-hydroxy-1,1-dimethylbutyl peroxyneoheptanoate) in combination (Japanese Pre-examination Patent Publication (KOKAI) No. 5-155910), but this technique is not satisfactory in fish eyes of the resulting vinyl chloride polymer.